#15676: mmap: add empty file check prior to offset check <- Previous patch was incomplete

Lib/test/test_mmap.py

@@ -469,17 +469,14 @@ class MmapTests(unittest.TestCase):
     def test_empty_file (self):
         f = open (TESTFN, 'w+b')
         f.close()
-        f = open(TESTFN, "rb")
+        with open(TESTFN, "rb") as f :
             try:
                 m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
                 m.close()
-            f.close()
                 self.fail("should not have been able to mmap empty file")
             except ValueError as e:
-            f.close()
                 self.assertEqual(e.message, "cannot mmap an empty file")
             except:
-            f.close()
                 self.fail("unexpected exception: " + str(e))
 
     def test_offset (self):

Modules/mmapmodule.c

@@ -1388,6 +1388,11 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
             }
 
             size = (((PY_LONG_LONG) high) << 32) + low;
+            if (size == 0) {
+                PyErr_SetString(PyExc_ValueError,
+                                "cannot mmap an empty file");
+                return NULL;
+            }
             if (offset >= size) {
                 PyErr_SetString(PyExc_ValueError,
                                 "mmap offset is greater than file size");